The present invention relates to a remotely actuated and locked hub clamping connector particularly useful in subsea applications for attaching subsea Christmas trees to marine wellheads.
Wellheads are typically used in oil and gas production wells, injection wells, and storage wells, and in geothermal wells which produce steam for power generation. Wellheads are generally affixed to the casing or tubing strings of a well to enable production. In subsea applications, marine wellheads provide the control mechanism between the well and the surface equipment. To control the flow of the well, subsea Christmas trees, also called flow assembly devices, are attached to the wellhead. These subsea Christmas trees consist of a series of valves and fittings which enable the flow of the well to be controlled and enable the well to be externally shut-in if needed.
Generally speaking, several clamping connectors have been developed for attaching drilling and piping components to wellheads. For example, the FastLock.TM., the FlangeLock.TM., and the FastClamp.TM. Connector's illustrated and discussed on page 404 of the 1996-1997 Cameron Catalog of Oil Field Equipment, represent three such clamping connectors. Each of these three connectors is intended to replace flanged connectors and reduce the amount of time necessary for makeup. In each of the above designs, the wellhead is clamped to the drilling or piping component by applying a torque to a series of studs and nuts. These correctors are typically used onshore, as the necessity of requiring a torque to a series of studs and nuts makes these connectors impractical for use in subsea applications.
An additional onshore clamping connector is disclosed in prior art reference U.S. Pat. No. 4,883.293 (Lawson). Unlike the above three connectors, the Lawson connector achieves its clamping force by pre-stressing a series of tension members. The tension members are stretched, or preloaded or pre-stressed, by the application of hydraulic force. While in this preloaded state, a block is placed in the gap created by the stretched members which prohibits relaxation of the tension members. The attempt by the tension members to return to their relaxed, unloaded state creates a clamping force that continues until the blocks are removed and the tension members are able to return to their relaxed state. The necessity of placing the block in the gap created by the stretched members makes the connector impractical for use in subsea applications.
When dealing with subsea applications, it becomes necessary to simplify or eliminate totally the diver's tasks in locking the connector in order to secure the connection between the drilling or piping components and the wellhead. For that purpose, hydraulically actuated collet connectors such as are shown in U.S. Pat. No. 4,730,853 (Gjessing) have been developed and utilized. These collet-type connectors are designed for high pressure and deepwater drilling operations. The collet connectors are secured over the mating sections of the drilling or piping components to be connected by use of hydraulic pressure. Because the hydraulic pressure can be controlled remotely, it is not necessary for the divers to lock the collet connector in order to secure the connection.
Although effective, these prior subsea, hydraulically controlled connectors have the disadvantage of being tall, bulky, and thus, expensive. For example, the connector disclosed in U.S. Pat. No. 4,730,853 (Gjessing) must be tall enough to house the vertical locking components. The height of that connector is therefore dependent on the amount of clamping force desired: the greater the required clamping force desired, the taller the piston. In turn, the hub of the components to be connected must be at least as tall as the connector's housing. Consequently, if that connector is used to attach a subsea Christmas tree to a marine wellhead, the lower hub of the Christmas tree must extend upwardly from the wellhead to at least the top of the connector housing. The net effect is that the overall height of the subsea Christmas tree is increased. The increased height becomes extremely significant when discussing deepwater, subsea connections. Any increased height increases the length of the moment arm that is acted upon. Thus, with increased height, the connector is subjected to increased forces and moments and the connector consequently has decreased reliability and durability.